We already knew smoking cigarettes was detrimental to our health. We also knew that second-hand smoke also causes the same harmful effects of smoking cigarettes. But did we know about third-hand smoke? According to a study conducted at the Lawrence Berkeley National Laboratory, researchers found that "third-hand smoke" is caused from smoking indoors. The leftover cigarette smoke that adheres to walls, furniture, carpet and clothing is believed to cause a much more serious threat than previously thought.